This invention relates to lens stop braking in automatic exposure cameras of stop value control type.
Automatic exposure cameras of lens stop control type are known in the art in which while the lens is stopped starting from its wide open state, the object's light passed through the lens aperture is measured, and when a suitable lens aperture is obtained With respect to predetermined shutter speed and film sensitivity, the lens stopping operation is stopped thereby automatically determining the lens aperture. If in this conventional automatic exposure camera the lens stopping operation is carried out by the use of the lens only, the operating time required to decrease the aperture starting from its wide open state is too short to lock the lens stop with high accuracy, and therefore a speed slowing mechanism is used in cooperation with an operating member operated by the lens stopping mechanism.
There is a problem, however, in using a speed slowing mechanism. The speed slowing mechanism may be employed to slow down the lens stopping operation so sufficiently that the lens stopping operation can be controlled with high accuracy. However, in the case where a suitable exposure cannot be obtained without decreasing the aperture to an aperture value almost equal to the minimum aperture value, the operating time is increased, that is, the period of time from the release to the shutter operation is increased, thus delaying the shutter operation. In order to overcome this difficulty, a method may be considered in which the effect of the speed slowing mechanism is alleviated to perform the lens stopping operation at higher rate. However, in this case, it is difficult to stop the lens stopping operation with high accuracy.
In addition, in general, the amount of movement required of a lens stop release plate is smaller in the minimum value side of the lens opening value. Therefore, even if a speed slowing mechanism which uniformly operates the lens stop release plate is employed, the quantity of light changes abruptly in the vicinity of the minimum lens opening. Accordingly, in the case when the lens stop release plate is operated at uniform rate, if the time required to change the lens aperture from its full open state to a predetermined value is not sufficiently long, the time per exposure value EV will be extremely short in the vicinity of the minimum lens opening, and therefore it is difficult to achieve the control with high accuracy. Again, in the case when the operating time is sufficiently long, the time from the release to the shutter operation becomes longer, which leads to a delay in shutter operation.
Furthermore, there is a delay time between the de-energization of a locking magnet and the locking operation thereof. Accordingly, it is necessary to de-energize the locking magnet slightly before the aperture is decreased to provide a desired brightness. Since the delay time is constant, if the variation time per EV is relatively long in the vicinity of the full lens opening and relatively short in the vicinity of the minimum lens opening, the locking operation cannot be effected as desired, thus causing an error in aperture control operation.